On Oxygen Activation at Rutile- and Anatase-TiO2

Article abstract of DOI:10.1021/acscatal.5b01562

The rutile form of titanium dioxide (r-TiO₂) usually shows a photocatalytic activity lower than that of anatase-TiO₂ (a-TiO₂). Nevertheless, there are numerous examples of sometimes unexpectedly high activity of r-TiO

Full text of DOI:10.1021/acscatal.5b01562

Subscriber access provided by The Chinese University of Hong Kong
Article
On oxygen activation at rutile- and anatase-TiO2
Marta Buchalska, Marcin Kobielusz, Anna Matuszek, Micha# Pacia, Szymon Wojty#a, and Wojciech Macyk
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01562 • Publication Date (Web): 05 Nov 2015
Downloaded from http://pubs.acs.org on November 11, 2015
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 10
ACS Catalysis
1
2
3
4
5
6
7
8
9
On oxygen activation at rutile- and anatase-TiO2
Marta Buchalska,† Marcin Kobielusz,† Anna Matuszek,†,‡ Michał Pacia,† Szymon Wojtyła† and
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Wojciech Macyk†*
† Faculty of Chemistry, Jagiellonian University in Kraków, Ingardena 3, 30ꢀ060 Kraków, Poland
‡ Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand
ABSTRACT: Rutile form of titanium dioxide (rꢀTiO2) usually shows a lower photocatalytic activity when compared to anataseꢀ
TiO2 (aꢀTiO2). Nevertheless, there are numerous examples of sometimes unexpectedly high activity of rꢀTiO2. This material may
appear particularly useful when a nonꢀcomplete and selective photocatalytic oxidation of organic substrates is required. Basing on
literature examples and our own studies we compare the photocatalytic activities of rꢀTiO2, aꢀTiO2 and rꢀTiO2/aꢀTiO2 composites.
Due to a significantly better oxygen adsorption at the surface of rꢀTiO2 and a lower redox potential of the excited electron, a more
•–
1
efficient O2 production takes place at the surface of rutile. As a consequence, also generation of O2 (involving O2 reduction and
the subsequent oxidation of superoxide) and reduction of H2O2 to OH– and HO• are favored at this material. Therefore rꢀTiO2 can be
considered as a particularly good photocatalyst for activation of molecular oxygen. On the other hand, aꢀTiO2 appears a stronger
oxidant. In its presence an efficient HO• generation (the result of water or surface hydroxyl groups oxidation) and H2O2 oxidation to
O2•– are observed.
KEYWORDS: rutile ꢀ anatase ꢀ titanium dioxide ꢀ oxygen activation ꢀ photocatalysis
processes carried out in acidic and neutral solutions.8 O2
•–
INTRODUCTION
participates also in early stages of degradation of phenolic
The differences between rutile and anatase structures of titaꢀ
compounds, while hydroxyl radicals react with the formed
intermediates leading to their complete mineralization.9
nium dioxide have a strong influence on their physicochemical
properties. RutileꢀTiO2 (rꢀTiO2) is a more stable and usually
less photoactive form of titanium dioxide. It exhibits a lower
electron mobility, a higher dielectric constant and a higher
density than anataseꢀTiO2 (aꢀTiO2).1 The point of zero charge
of TiO2 increases with the content of rutile in the sample.2 The
rates of charge recombination are lower in the case of rꢀTiO2
materials.3
Generation of reactive oxygen species at TiO2
surface
Oxygen is an oxidant characterized by its unique triplet
3
ground state. A total reduction of O2 is a fourꢀelectrons proꢀ
cess with water as an endꢀproduct (E° = 1.23 V vs. SHE). The
multiplicity of electron donor should also amount 3. Due to
3
the scarcity of such donors O2 remains relatively inactive in
The difference between aꢀTiO2 and rꢀTiO2 reflects also in
interactions of these materials with dioxygen. Molecular oxyꢀ
gen interacts weakly with a stoichiometric TiO2 surface, but
shows a high affinity to oxygen vacancies.4 Under cryogenic
conditions three oxygen molecules may interact with one
vacancy.5 Opposite to rutile, in anatase the oxygen vacancies
are localized under the most stable (101) facets.6 In conseꢀ
quence, the interaction of oxygen molecules with the (101)
surface of anatase is weaker than with the most stable (110)
facet of rutile.6 Therefore significant differences in the concenꢀ
tration of adsorbed oxygen are observed in the case of both
forms of TiO2. Adsorption of O2 is associated with an electron
transfer from the surface defects and results in formation of
superoxide anion.4, 7 This reactive oxygen species presents a
relatively low reactivity towards organic compounds oxidaꢀ
tion, due to its disproportionation to molecular oxygen and
oxidation reactions. The processes of partial reduction of
oxygen result in formation of reactive oxygen species (ROS).
This group of reactive products includes superoxide anion
•–
(O2 ), its protonated form – hydroperoxyl radical (HOO•),
hydrogen peroxide (H2O2) and hydroxyl radical (HO•) being
one of the strongest oxidants. Also the nonꢀradical form, sinꢀ
glet oxygen 1O2, belongs to the group of ROS.10
Several mechanisms lead to the formation of reactive oxyꢀ
gen species. Reduction of molecular oxygen by electrons, eCB
–
,
with superoxide anion formation, and water oxidation by
holes, hVB+, with generation of hydroxyl radicals, are the main
primary redox processes ruling the photocatalysis at TiO2 in
aqueous environments.11 Further reduction and protonation of
superoxide anion leads to hydroperoxyl radical, hydrogen
peroxide, hydroxyl radical and water. Therefore hydroxyl
radicals can be generated in two different pathways, as the
•–
H2O2. However, O2 plays an important role in the oxidation
ACS Paragon Plus Environment

ACS Catalysis
Page 2 of 10
result of either oxidation of water or a multistep reduction of
amount ca. 0.4 eV, therefore the photogenerated electrons
should move efficiently from rutile to anatase particles.
oxygen.12
1
2
3
4
5
6
7
Singlet oxygen is generated either through a direct energy
transfer from the excited state of a photosensitizer (usually an
organic dye) to oxygen, or as a consequence of consecutive
redox reactions of O2. The Nosaka’s group put forward the
1
hypothesis, that O2 is formed in the presence of TiO2 accordꢀ
ing to the latter mechanism: molecular oxygen is reduced to
superoxide radical anion, which is then oxidized by holes
forming singlet oxygen.13 This process can also occur in the
presence of sensitized TiO2 materials.14
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Photoactivity of rutile/anatase composites
Unusually high photocatalytic activities of TiO2 have been
reported for anatase/rutile composites. A good example is the
P25 material produced by Evonik consisting of 70ꢀ80% of
anatase and 30ꢀ20% of rutile. Since the photoreactivity of this
composite is, in general, higher than that observed for bare aꢀ
TiO2 and rꢀTiO2, P25 is often used as a reference material,
although homogeneity of P25 is disputable. A low homogeneiꢀ
ty of P25 leads to considerable differences in photocatalytic
activity of various samples of this reference material.15 The
generally reported high photoactivity of P25 is very likely
associated with charge transfer taking place between particles
of different phases. At least two different pictures of these
processes exist in literature.16 Ohno et al. showed that in a
mixture of large rutile crystals and smaller anatase particles
the rutile phase is photoexcited. Although the conduction band
edge of aꢀTiO2 is considered to be characterized by ca. 0.2 eV
higher energy than the corresponding CB edge of rꢀTiO2,16d, 17
electrons are transferred from rꢀTiO2 to surface states of aꢀ
TiO2, from where they are consumed in oxygen reduction (Fig.
1). Photogenerated VB holes of rutile can oxidize an organic
pollutant directly, or indirectly, through hydroxyl radicals.16a
This theory is supported by EPR studies of Thurnauer and
coworkers who suggested that electrons excited within rutile
crystals are stabilized within lower energy states of anatase.16b,
16c On the other hand, there are evidences of the electron transꢀ
fer in the opposite direction, from the conduction band of
anatase to CB of rutile,16dꢀf in accordance with the relative
energies of CB edges of aꢀTiO2 and rꢀTiO2 presented in Fig. 1.
Trapping of excited electrons within the rꢀTiO2 component of
the composite was supported by the results of EPR
measurements.16e, 16f
Figure 1. Two possible directions of electron transfer between aꢀ
TiO2 and rꢀTiO2 in composite materials: from rutile to anatase16aꢀc
and from anatase to rutile.16dꢀf
Phase composition vs. photoactivity
Numerous studies have been dedicated to the influence of
18
the anatase/rutile ratio on the photocatalytic activity.16a,
Usually a high content of more reactive anatase does not guarꢀ
antee the high activity of the whole composite. Ohno et al.16a
studied the photocatalytic degradation of naphthalene in the
presence of aꢀTiO2 calcined at different temperatures. The
material annealed at 800°C, containing 90% of rꢀTiO2, apꢀ
peared the most active sample. This material was slightly
better than P25 and nearly 3 times more active than the startꢀ
ing aꢀTiO2.16a
Similar results were observed by other researchers in the
processes of various compounds photodegradation,
e.g. methylene blue,18 Acid Red 1,19 phenol,20 or
acetaldehyde21 (the gas phase oxidation in the last case). Phoꢀ
tocatalysts calcined at various temperatures were photoactive
until both aꢀTiO2 and rꢀTiO2 were present in the samples.
Apparently, the optimal composition of the photocatalyst
cannot be generalized since it depends inter alia on the subꢀ
strate that is oxidized in the photocatalytic process.
Substrate nature vs. photoactivity
As often demonstrated, several photocatalytic reactions can
be realized in the presence of rꢀTiO2, although usually the
efficiencies of these processes are lower than those observed
for aꢀTiO2. For example, the efficiency of 2ꢀpropanol oxidaꢀ
tion in the presence of irradiated rꢀTiO2 was found to reach ca.
40% of that measured for aꢀTiO2.22 A significantly lower phoꢀ
toactivity of rꢀTiO2 is, however, not a general rule. Tayade et
al.23 studied a photocatalytic degradation of various organic
compounds (acetophenone and nitrobenzene among others) in
the presence of both aꢀTiO2 and rꢀTiO2. Although aꢀTiO2 in
most cases showed a higher photoactivity, rꢀTiO2 also apꢀ
peared active. In particular, the degradation rate of acetopheꢀ
In both cases the charge separation would lead to an inꢀ
creased lifetime of the electronꢀhole pair and a higher photoacꢀ
tivity of the rꢀTiO2/aꢀTiO2 composite. Thurnauer points also at
the distorted fourꢀcoordinated interfacial sites localized at the
interface between aꢀTiO2 and rꢀTiO2, which might act as cataꢀ
lytic “hot spots” decreasing the activation barrier of the elecꢀ
tron transfer processes.16b, 16c
Recent calculations of Scanlon et al. revealed the lower poꢀ
tential of the CB edge of rutile than of anatase, opposite to the
situation shown in Fig. 1.16a The band alignment should
ACS Paragon Plus Environment

Page 3 of 10
ACS Catalysis
none in the presence of rutile was almost as high as at anatase.
Similar rates were observed also for nitrobenzene, however,
1
2
3
4
5
6
7
8
degradation of this compound proceeded generally much
slower than the decomposition of acetophenone, due to a highꢀ
er stability of the former compound. Considering structures of
acetophenone and nitrobenzene it can be noticed, that oxidaꢀ
tion of these compounds may also be initiated by a Dielsꢀ
Alder addition of singlet oxygen. A possible role of singlet
oxygen in photocatalytic oxidation reactions will be discussed
hereafter.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Presented works confirm the varying photoactivity of rꢀTiO2.
Depending on the substrate structure it may be almost inactive
or more active than aꢀTiO2. This leads to a conclusion, that
either adsorption of substrates at rꢀTiO2 and aꢀTiO2 differs
dramatically or the photocatalytic oxidation in the presence of
these two materials follows various pathways and, apparently,
involves various reactive oxygen species.
The influence of H2O2
The efficiency of hydroxyl radicals and superoxide anions
formation can be very different when comparing rꢀTiO2 and aꢀ
TiO2 (vide infra).24 In addition, the presence of H2O2 can draꢀ
Figure 2. Mechanism of hydroxyl radical generation at the surꢀ
matically influence the generation of HO• and O2 , differently
•–
face of rꢀTiO2 involving η2ꢀperoxide structure.24
at rꢀTiO2 and aꢀTiO2. For instance, the production of hydroxyl
radicals at rutile surface is very inefficient, however, generaꢀ
tion of this reactive species in the presence of H2O2 occurs
with high yields at rꢀTiO2 and a,rꢀTiO2 composites (10ꢀ20% of
rutile), but not at aꢀTiO2.24 Moreover, in aqueous solutions a
higher stability of O2•– is offered by the rutile surface, whereas
at aꢀTiO2 superoxide radical is efficiently consumed to proꢀ
duce hydrogen peroxide. The phenomenon of a significant
difference in activity of rꢀTiO2 in the presence and absence of
H2O2 was explained by different adsorption properties of both
materials.24 Only in the case of rꢀTiO2 the η2ꢀperoxide strucꢀ
ture can be formed (both oxygen atoms of H2O2 are bound to
one Ti site, Figure 2), leading to the formation of hydroxyl
radicals from H2O2.24
Partial oxidation reactions
Benzene oxidation in air showed that 60ꢀ80% of the product
obtained in the presence of rꢀTiO2 material contains –OH
groups originating from O2 molecules. In the case of anatase
70ꢀ90% of formed phenol contained –OH groups originating
from water. The surface oxide and peroxide radicals could be
the source of oxygen as proposed in Fig. 3. Further oxidation
steps of benzene (to CO2 as the final product) were observed
with a higher efficiency in the presence of aꢀTiO2, while at rꢀ
TiO2 large amounts of phenol were formed.26
Similar conclusions can be drawn from the results of reꢀ
search done by the Ohno’s group. Photooxidation of adamanꢀ
tine was achieved in the presence of both crystalline forms of
TiO2.25 Oxidation of adamantine by hydroxyl radicals proꢀ
ceeded selectively leading to 1ꢀadamantanol, 2ꢀadamantanone
and 2ꢀadamantanol as main products.25a The most significant
difference could be observed in the efficiency of 2ꢀ
adamantanol generation – it was the lowest in the presence of
rꢀTiO2 and the highest in the presence of aꢀTiO2. A high yield
of 2ꢀadamantanol generation was observed when H2O2 photolꢀ
ysis was done in the presence of adamantine. At rꢀTiO2 the
reaction strongly accelerated upon addition of hydrogen perꢀ
oxide.25b These results are in accordance with the enhanced
generation of hydroxyl radicals at rꢀTiO2 in the presence of
H2O2.
Figure 3. The mechanism of benzene hydroxylation in the presꢀ
ence of anatase and rutile with water (A) and O2 (B) as the oxygen
source. The contribution of both pathways in phenol formation in
the presence of rꢀTiO2 and aꢀTiO2 (C).26
ACS Paragon Plus Environment

ACS Catalysis
Page 4 of 10
products at the surface of photocatalysts, and ii) various mechꢀ
1
2
3
4
5
6
7
8
anisms of reactive oxygen species generation and their further
transformations. In our studies we focus on the differences in
oxygen activation at rꢀTiO2 and aꢀTiO2. For our studies we
have selected four commercial TiO2 materials: rꢀTiO2 (TR,
Tronox), nanoꢀrꢀTiO2 (nꢀrꢀTiO2, Sachtleben), aꢀTiO2 (AKꢀ1,
Tronox) and P25 (Evonik).
Other studies on partial oxidation of methoxybenzyl alcohol
to pꢀanisaldehyde in the presence of various forms of TiO2
showed that the selectivity of the process differs for rutile,
anatase and their composites.27 RꢀTiO2 (both, prepared by
authors and a commercial material) appeared nearly twice as
efficient in the aldehyde generation as aꢀTiO2. Moreover, the
rate of this reaction in the presence of P25 was lower than in
the case of rꢀTiO2 or aꢀTiO2. Rutile materials showed the
highest selectivity of the aldehyde formation. AꢀTiO2 and a,rꢀ
TiO2 photocatalyzed efficiently a complete oxidation of alcoꢀ
hol. This effect can be explained by a weaker pꢀanisaldehyde
bonding to the rutile surface than to anatase or P25, therefore a
further oxidation of pꢀanisaldehyde was hindered due to its
diffusion from the surface of the catalyst.
9
EXPERIMENTAL SECTION
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Materials: Evonik P25 (70% anatase/30% rutile; specific
surface area of 58 m2/g), anatase aꢀTiO2 – Tronox AKꢀ1
(>97% anatase; 90 m2/g), rutile rꢀTiO2 – Tronox TR (>99.5%
rutile; 5.5 m2/g), nanocrystalline rutile, nꢀrꢀTiO2 – Sachtleben
(“NanoꢀRutil” E3ꢀ692ꢀ011ꢀ009, >99% rutile; 122 m2/g).
BrunauerꢀEmmettꢀTeller (BET) measurements were perꢀ
formed using an ASAP 2010 (Micromeritics) instrument. The
samples were heated at 250°C for 3 h under vacuum prior to
the measurements. The specific surface area of the samples
was estimated using the BET equation (nitrogen adsorption at
77 K). Measurements of hydrodynamic diameter were perꢀ
formed using a Malvern Zetasizer NanoZS instrument.
Also a selective oxidation of benzyl alcohol to benzaldehyde
was achieved in the presence of rutile nanorods.28 The selecꢀ
tivity of the process was very high – benzaldehyde was proꢀ
duced with a selectivity of 99%. A high density of surface
hydroxyl groups was the key issue to reach a high selectivity
of the photocatalyst. Desorption of aldehyde was facilitated by
a high surface hydrophilicity (the consequence of a high densiꢀ
ty of surface hydroxyl groups). In the case of aꢀTiO2 adsorpꢀ
tion of benzaldehyde was better and in consequence its further
oxidation to benzoic acid took place.28
Surface conditioning: All materials were heated under vacꢀ
uum conditions at 150°C for 5 h. Afterwards the samples were
stored in argon atmosphere.
Oxygen adsorption: PyroScience FireStingO2 oxygen meter
was used to analyze dissolved oxygen concentration in all
measurements. In order to determine the amount of oxygen
adsorbed at tested materials the previously reported procedure
was applied.31 Equal volumes of air saturated water (100 µL)
was injected (11 times) into argon saturated titanium dioxide
suspension (5.0 mL, 1 g/dm3). The injections were done in 5
min intervals. After injection of the last air saturated water
dose the oxygen concentration was monitored for 15 minutes,
until a stable reading from the sensor was achieved. The difꢀ
ference in dissolved oxygen concentration measured for disꢀ
tilled water and sample suspensions was used to calculate the
amount of oxygen adsorbed at the surface of particles.
Very interesting results of Lꢀalanine photooxidation were
presented by Matsushita et al.29 The distribution of products of
this reaction (pyruvic acid and acetamide), as well as the reacꢀ
tion yield, depended strictly on the crystal structure of the
photocatalyst. RꢀTiO2 photocatalyzed a direct oxidation of Lꢀ
alanine to pyruvic acid. In the presence of aꢀTiO2 acetamide
was formed through condensation of acetaldehyde with amꢀ
monia, the products of Lꢀalanine decomposition. The products
distribution in the presence of P25 was similar to that observed
for aꢀTiO2.29
Irradiation of the naphthalene solution (in water/acetonitrile
99:1) in the presence of TiO2 also led to formation of various
products, depending on crystal structure of the applied photoꢀ
catalyst. In the first step of reaction a partial oxidation of aroꢀ
matic ring took place, similarly to the process of benzene
oxidation,26a with generation of hydroxynaphthalene in the first
step. In the presence of hydroxyl radicals the substitution of
the next hydrogen atom with hydroxyl group occurred at the
neighboring position. As the result of further oxidation steps
the ring opening with generation of e.g. phthalic anhydrate
was observed, in particular in the presence of rꢀTiO2. A further
oxidation to CO2 as the final product was more efficient at aꢀ
TiO2. The first steps of the photocatalytic oxidation of naphꢀ
thalene were similar to those valid for benzene oxidation (Fig.
3).16a, 30
Photoelectrochemical measurements: Photoelectrochemiꢀ
cal measurements were carried out using Autolab
PGSTAT302N potentiostat and XBO150 xenon lamp with a
monochromator (Instytut Fotonowy). Measurements have been
done in the threeꢀelectrode cell using platinum wire and
Ag/AgCl as a counter and reference electrodes, respectively. A
thin layer of material on ITO foil (resistivity: 60 ꢁ/sq) was
used as a working electrode. The electrodes were placed in a
quartz cuvette filled with 0.1 mol/dm3 KNO3 (≥99.0%, Sigmaꢁ
Aldrich) solution (pH = 6.1) in pure water (MilliꢀQ water) as
an electrolyte. The measurements have been done after saturaꢀ
tion of the electrolyte with oxygen or argon (oxygen free conꢀ
ditions). Irradiation was done at 380 nm. Photocurrent generaꢀ
tion was measured in the potential range of –0.2ꢀ1 V vs.
Ag/AgCl.
The presented state of art shows, that the differences in phoꢀ
toactivity of rꢀTiO2 and aꢀTiO2 can be attributed to two main
factors: i) different adsorption of substrates, intermediates and
ACS Paragon Plus Environment

Page 5 of 10
ACS Catalysis
Redox properties of the materials: Redox properties of the cuvette was placed in the spectrofluorometer Fluorologꢀ3
1
2
3
4
5
6
7
8
materials were determined using spectroelectrochemical
method which is based on electrochemical measurements
combined with UVꢀvis diffuse reflectance spectroscopy.32 The
changes in reflectance were recorded at λ = 780 nm by Perkin
Elmer UVꢀVis Lambda 12 spectrometer equipped with a 5 cm
dia. integrating sphere. The electrochemical measurements
were carried out in threeꢀelectrode cell, with platinum wire
and Ag/AgCl as a counter and reference electrodes, respecꢀ
tively. Titanium dioxide materials were previously ground in a
mortar and suspended in acetonitrile. Working electrodes were
prepared by casting of tested materials at the surface of platiꢀ
num foil (ca. 2 cm × 1 cm). Afterwards the working electrodes
were dried at approx. 100°C. In this way opaque films of the
materials were formed on Pt plate. The electrodes were placed
in a cuvette with a quartz window filled with 0.1 mol/dm3
LiClO4 (99.99% trace metals basis, Aldrich) solution in aceꢀ
tonitrile. Oxygen was thoroughly removed from the electrolyte
by purging with argon before (15 min) and during the experiꢀ
ments. The cuvette was placed in front of the sphere, facing
the working electrode (platinum foil with deposited TiO2)
towards the light beam. The potential control was provided by
the electrochemical analyzer BioꢁLogic SPꢀ150 (scan rate 0.5
mV/s).
(Horiba Jobin Yvon). The suspension was stirred with a magꢀ
netic stirrer during the measurement and irradiated continuꢀ
ously at 290 nm. The phosphorescence of singlet oxygen was
recorded by a NIR photomultiplier tube with InP/InGaAsP
photocathode (Hamamatsu) equipped with an iHR320 imaging
spectrometer. Singlet oxygen phosphorescence spectra were
collected within 1000 cycles and then averaged.
9
Synthetic air, oxygen and argon (99.9999%) were purchased
from Air Products.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. The process of TAOH formation in the reaction of
terephthalic acid (TA) with hydroxyl radicals.33
RESULTS AND DISCUSSION
Materials
Photocatalytic activity: The photocatalytic activity of the
materials was examined by irradiation of the suspensions of
TiO2 (20 mL, 1 g/dm3) in the solution of terephthalic acid
(Aldrich, 98%) (6×10–3 mol/dm3 in aqueous solution of 0.02
mol/dm3 NaOH, pH = 11) with an XBOꢀ150 xenon lamp (Inꢁ
stytut Fotonowy). A water filter (10 cm) with 0.1 mol/dm3
solution of CuSO4 was used to absorb NIR and IR radiation,
together with a 320 nm cutꢀoff filter. Air was slowly bubbled
through the suspension to avoid sedimentation as well as to
ensure a constant access of oxygen. Samples of 2 mL were
collected during irradiation and filtered through CME filters
(pore size of 0.22 ꢀm). The applied TiO2 concentration enaꢀ
bled a direct comparison of photocatalytic activity of tested
sample (a further increase of TiO2 concentration did not result
in the reaction acceleration).
The materials selected for our studies are both nanocrystalꢀ
line aꢀTiO2 (particle size 30ꢀ50 nm) and nꢀrꢀTiO2 (30x70 nm),
as well as microcrystalline rꢀTiO2 (120ꢀ150 nm). AꢀTiO2 and
nꢀrꢀTiO2 are characterized by similar specific surface areas (90
and 122 m2/g, respectively), whereas rꢀTiO2 shows a much
less developed surface (5.5 m2/g). NꢀrꢀTiO2 forms spherical
aggregates with the diameter of ca. 10 ꢀm, as confirmed by
SEM measurements (data not shown). Other materials form
less ordered aggregates. In water, at pH ca. 6, aꢀTiO2 and nꢀrꢀ
TiO2 form aggregates of a similar hydrodynamic diameter of
(0.5ꢀ0.6 ꢀm), while aggregates of rꢀTiO2 and P25 are smaller
(ca. 100 nm; data not shown). Photocatalytic activity of TiO2
can be influenced by various contribution of different facets in
the overall surface area or by different terrace/step configuraꢀ
tions.34 All selected materials have irregular shapes and thereꢀ
fore present in some sense “average” anatase and rutile samꢀ
ples. To avoid the influence of adsorbed species on the studied
properties all materials were heated at 150°C under vacuum
for 5 h and stored in argon atmosphere.
The process of terephthalic acid (TA) conversion to hydroxꢀ
yterephthalic acid (TAOH) was used to compare efficiencies
of hydroxyl radicals generation (Fig. 4).33 Photocatalysts were
irradiated in TA solution (3 mM TA in 10 mM NaOH) for 30
min. Samples were collected in 5 min intervals. In the reaction
of nonꢀfluorescent TA with hydroxyl radicals the formation of
TAOH can be monitored by emission spectra measurements.
TAOH shows a broad emission band at λmax = 425 nm when
excited at λexc = 315 nm. Fluorescence spectra were measured
using FluoroLogꢀ3 (Horiba JobinYvon) spectrofluorometer in
1 cm quartz cuvette.
Oxygen adsorption
The amounts of oxygen adsorbed at rꢀTiO2 and nꢀrꢀTiO2 are
1.5 and 2.5 times higher than at aꢀTiO2, respectively (red bars
in Fig. 5). The results of measurements were recalculated to
the amount of oxygen adsorbed at 1 g and 1 m2 of the photoꢀ
catalyst. The surface concentrations of adsorbed oxygen are
significantly higher for rutile samples than for anatase (Table
1). These differences are in agreement with previous reports
on oxygen adsorption at TiO2 materials.5b, 35
Singlet oxygen detection: The materials (0.1 g dm–3) were
suspended in CD3OD (2.5 mL, Aldrich) in a quartz cuvette
equipped with a rubber septum. Oxygen was bubbled through
the suspension for 5 minutes prior to the measurement. The
Photoelectrochemistry
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 10
Photocurrents generated at the electrodes covered with rꢀ The net photocurrent is the sum of anodic and cathodic phoꢀ
1
2
3
4
5
6
7
8
TiO2 and aꢀTiO2 were measured as a function of wavelength
and potential (Fig. 6). For both rꢀTiO2 and aꢀTiO2 anodic and
cathodic photocurrents were recorded. The anodic photocurꢀ
rent is generated when valence band holes oxidize water,
while electrons are transferred to the ITO electrode through
the TiO2 film. The cathodic photocurrent is generated when
electrons reduce an adsorbed electron acceptor (e.g. molecular
oxygen), while holes migrate to the electrode.36
tocurrents. In the absence of any electron acceptor (O2) the
anodic photocurrents prevail, while cathodic photocurrents
dominate in the presence of oxygen. Particularly high cathodic
photocurrents can be observed for rꢀTiO2 at negative potentials
(Fig. 6). When the electrode is covered with aꢀTiO2 the influꢀ
ence of oxygen is insignificant. This observation points at a
much more efficient oxygen reduction at rꢀTiO2 than at aꢀ
TiO2. It is also consistent with the higher oxygen availability
at the surface of rutile, as confirmed in the study of oxygen
adsorption (Fig. 5). Recently, using spin trapping and EPR
spectroscopy Li et al. confirmed more efficient O2•− photogenꢀ
eration at rꢀTiO2 compared to aꢀTiO2.37
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
dissolved oxygen
adsorbed oxygen
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
4.6
4.5
4.4
4.3
4.2
4.1
4.0
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3.0
Redox properties of the materials
To study redox properties of TiO2 we have applied spectroeꢀ
lectrochemical measurements developed recently in our laborꢀ
atory.32 The electrochemical reduction of TiO2 generates TiIII
centers absorbing at 780 nm, localized beneath the bottom of
the conduction band. Reflectance changes measured at 780 nm
as a function of the electrode potential (TiO2 deposited at
platinum electrode) and transformed to KubelkaꢀMunk (KM)
function are presented in Fig. 7. The deflection points on recꢀ
orded curves correspond to the onset reduction potentials at
which trapping of electrons occurs. The last deflection points
correspond to potentials of the conduction band edges (–1.14,
–1.24 and –1.53 V for aꢀTiO2, nꢀrꢀTiO2 and rꢀTiO2, respectiveꢀ
ly). The CB edge of rꢀTiO2 is therefore localized at lower
potentials than the CB of aꢀTiO2, what is in contradiction to
previous reports,16b, 16c but in accordance with calculations of
Scanlon et al. 16h In the case of P25 the last deflection point is
close to that of nꢀrꢀTiO2. Other values of potentials marked
with arrows in Fig. 7 can be attributed to electronic states
acting as traps. Our results show that the density of states close
to the conduction band is higher for aꢀTiO2 than for rꢀTiO2.
Similar results were reported by Jankulovska et al.38 Moreoꢀ
ver, anatase is characterized be remarkable deeper traps. Elecꢀ
trons trapped within the bandgap of aꢀTiO2 offer much worse
reduction properties than electrons trapped within rꢀTiO2 and
nꢀrꢀTiO2. The results of the measurements for all materials are
summarized in Figure 8.
water
r-TiO2
a-TiO2
n-r-TiO2
Figure 5. Oxygen adsorption at the surface of anatase and rutile
materials.
Table 1. Amounts of oxygen adsorbed at aꢀTiO2, rꢀTiO2 and
nꢀrꢀTiO2 surfaces (±5%).
dissolved
oxygen
specific
surface
adsorbed oxygen
concentration
Material
concentration area
[nmol/m2] [µmol/g]
[µmol/dm3]
[m2/g]
rꢀTiO2
3.72
3.12
4.04
5.5
122
90
5280
390
29
48
19
nꢀrꢀTiO2
aꢀTiO2
210
The values of CB potentials and electronic states measured
in acetonitrile differ from those measured in water. The nonꢀ
aqueous electrolytes containing alkali or alkaline earth cations
shift the flat band potential.39 However, the measurements
should be done in nonꢀaqueous electrolytes to avoid water
reduction and to lower the noise/signal ratio.39ꢀ40
Figure 6. Photocurrent generation (λ = 380 nm) in the presence of
aꢀTiO2 and rꢀTiO2 electrodes. The measurements were carried out
in aqueous 0.1 M KNO3 solution saturated with oxygen or argon.
ACS Paragon Plus Environment

Page 7 of 10
ACS Catalysis
a-TiO2
1
2
3
4
5
6
7
8
P25
n-r-TiO2
120
100
80
60
40
20
0
P25
a-TiO2
r-TiO2
n-r-TiO2
r-TiO2
0.2 a.u.
9
10
11
12
13
14
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
/ V
Ag/AgCl
vs.
Potential
0
5
10
15
20
25
30
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7. Changes of KM measured at 780 nm as a function of
the applied potential for aꢀTiO2, rꢀTiO2, nꢀrꢀTiO2 and P25 deposꢀ
ited at the surface of platinum plate. The measurements were
carried out in deoxygenated 0.1 mol/dm3 LiClO4 solution in aceꢀ
tonitrile.
Irradiation time / min
Figure 9. Photogeneration of TAOH in the process of photocataꢀ
lytic oxidation of TA in the presence of rꢀTiO2, nꢀrꢀTiO2, aꢀTiO2
and P25. Irradiation > 320 nm.
Singlet oxygen generation was followed by monitoring its
phosphorescence (Fig. 10). Under the applied conditions no
phosphorescence could be observed in the presence of aꢀTiO2,
1
while all other samples appeared photoactive in O2 generaꢀ
tion. A low intensity observed for nꢀrꢀTiO2 results to some
extent from a fast sedimentation of this material (nanoparticles
form aggregates of ca. 10 ꢀm of diameter). A better evolved
signal was observed for rꢀTiO2, while the strongest one was
recorded for P25 material. A higher yield of singlet oxygen
formation at rꢀTiO2 compared to aꢀTiO2 is justified. Our reꢀ
1
sults are in accordance with the mechanism of O2 generation
proposed by Nosaka, which involves formation of superoxide
1
anion and its reoxidation to O2 by holes.13a The yield of sinꢀ
Figure 8. The energy diagram close to the conduction band edges
of aꢀTiO2, rꢀTiO2, nꢀrꢀTiO2 and P25. Potential values vs. Ag/AgCl
electrode. CB of rꢀTiO2 is marked in blue, CB of aꢀTiO2 – in
green.
glet oxygen generation is influenced by several factors, includꢀ
ing the concentration of adsorbed oxygen.
a-TiO2
n-r-TiO2
Photocatalytic activity
Interesting conclusions on photoactivity of aꢀTiO2 and rꢀ
TiO2 can be drawn from the experiments of terephthalic acid
(TA) oxidation and singlet oxygen generation. The efficiency
of hydroxyl radicals generation is reflected by the progress of
TAOH formation (Fig. 9). As expected, rutile samples show
the lowest photoactivity in this test due to inefficient hydroxyl
radical generation in the process of water oxidation at the most
stable (110) facet of rꢀTiO2.41 The alternative mechanism of
hydroxyl radicals generation at rꢀTiO2 involves threeꢀstep
reduction of oxygen molecule. Oxidation of TA to TAOH, and
thus generation of hydroxyl radicals, is much faster when aꢀ
TiO2 or the rꢀTiO2/aꢀTiO2 composite are applied.
r-TiO2
P25
1220 1240 1260 1280 1300 1320
Wavelength / nm
Figure 10. Phosphorescence of singlet oxygen photogenerated in
the presence of rꢀTiO2, nꢀrꢀTiO2, aꢀTiO2 and P25. Irradiation >
320 nm.
ACS Paragon Plus Environment

ACS Catalysis
Page 8 of 10
facets and terrace/step configurations, however, already a
CONCLUSIONS
1
2
3
4
5
6
7
8
comparison of the macroscopic properties of rꢀTiO2 and aꢀ
TiO2, in particular their redox characteristics, helps to underꢀ
stand the differences between rꢀTiO2, aꢀTiO2 and rꢀTiO2/aꢀ
TiO2.
As often proved, rꢀTiO2 cannot be considered as an inactive
material in photocatalytic processes. The activity of this crysꢀ
talline form of TiO2 was described in several types of chemiꢀ
cal reactions. RꢀTiO2 may appear more suitable for generation
of the products of nonꢀcomplete substrate oxidation and can be
considered as a photocatalyst of reactions requiring a higher
selectivity. In particular, the reactions involving superoxide
radicals or singlet oxygen are favored at rꢀTiO2.
AUTHOR INFORMATION
9
Corresponding Author
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Spectroelectrochemical studies enabled redox characterizaꢀ
tion of rꢀTiO2 and aꢀTiO2. The results are in contradiction with
the commonly accepted picture presented in Fig. 1. Apparentꢀ
ly, the reduction of aꢀTiO2 is easier than the reduction of rꢀ
TiO2, therefore the electron transfer from the conduction band
of rꢀTiO2 to aꢀTiO2 should be expected rather than the opposite
process. Our results are in accordance with the latest theoretiꢀ
cal studies of the band structures of TiO2 crystalline forms.16h,
* eꢀmail: macyk@chemia.uj.edu.pl
ACKNOWLEDGEMENTS
The authors thank Prof. Horst Kisch and Prof. Bunsho Ohtani for
valuable discussions. The studies were realized within the „Actiꢀ
vation of small molecules in photocatalytic systems” project of
TEAM program (TEAM/2012ꢀ9/4) supported by the Foundation
for Polish Science, coꢀfinanced by European Union, Regional
Development Fund. The part of the work was supported by Polish
Ministry of Science and Higher Education (grant no N N204
016739). The equipment was purchased thanks to the financial
support of the European Regional Development Fund in the
framework of the Polish Innovation Economy Operational Proꢀ
gram (contract no. POIG.02.01.00ꢀ12ꢀ023/08).
38
A comparison of redox properties of rꢀTiO2 and aꢀTiO2
•−
helps to understand the differences in yields of O2 and HO•
generation at these materials. Since excited rꢀTiO2 is a signifiꢀ
cantly stronger reducer and a weaker oxidant than aꢀTiO2,
superoxide ions are easily formed at rꢀTiO2 as the result of O2
reduction and at aꢀTiO2 as the result of H2O2 oxidation. On the
other hand, HO• radicals are produced efficiently at rꢀTiO2
through H2O2 reduction to OH– and HO• and at aꢀTiO2 through
H2O oxidation. Redox properties of both polymorphs of titaniꢀ
um dioxide may, in addition to other factors (like differences
in charge mobility, lifetimes of photogenerated charges, etc.),
account for their various photocatalytic properties.
REFERENCES
1.
(a) Carp, O.; Huisman, C. L.; Reller, A., Prog. Solid State
Chem. 2004, 32, 33ꢀ177; (b) Laboueiau, A.; Earl, W. L., Chem. Phys.
Lett. 1997, 270, 278–284; (c) Sekiya, T.; Ichimura, K.; Igarashi, M.;
Kurita, S., J. Phys. Chem. Solids 2000, 61, 1237ꢀ1242; (d) Mo, S.ꢀD.;
Ching, W. Y., Phys. Rev. B 1995, 51, 13023ꢀ13032.
2.
Appl. Mater. Interfaces 2013, 5, 6247ꢀ6254.
3. Wang, X. L.; Kafizas, A.; Li, X. O.; Moniz, S. J. A.; Reardon,
Kinsinger, N. M.; Dudchenko, A.; Wong, A.; Kisailus, D., ACS
The results of our studies confirm a considerable better oxyꢀ
gen adsorption at the surface of rutile (Fig. 5). This favors
P. J. T.; Tang, J. W.; Parkin, I. P.; Durrant, J. R., J. Phys. Chem. C
2015, 119, 10439–10447.
•−
4.
5.
Li, Y. F.; Selloni, A., J. Am. Chem. Soc. 2013, 135, 9195–9199.
(a) Diebold, U., Surf. Sci. Rep. 2003, 48, 53–229; (b) Pang, C.
reduction of oxygen to O2 in the presence of this form of
TiO2 (evidenced by high cathodic photocurrents recorded for rꢀ
TiO2 in the presence of oxygen, Fig. 6). Phosphorescence
measurements show that the efficiency of singlet oxygen genꢀ
eration is higher in the presence of rꢀTiO2 than of aꢀTiO2. This
observation is understandable taking into account a better O2
adsorption and its more efficient reduction at rꢀTiO2. In conseꢀ
quence, rꢀTiO2 can be recommended as a photocatalyst of
dioxygen activation, i.e. when generation of superoxide radiꢀ
cals or singlet oxygen is required.
L.; Lindsay, R.; Thornton, G., Chem. Soc. Rev. 2008, 37, 2328ꢀ2353.
6. Li, Y.ꢀF.; Aschauer, U.; Chen, J.; Selloni, A., Acc. Chem. Res.
2014, 47, 3361–3368.
7.
Wang, Z. T.; Deskins, N. A.; Lyubinetsky, I., J. Phys. Chem.
Lett. 2012, 3, 102–106.
8.
Ndong, L. B. B.; Gu, X.; Lu, S.; Ibondou, M. P.; Qiu, Z.; Sui,
Q.; Mbadinga, S. M.; Mu, B., Chem. Eng. Sci. 2015, 123, 367–375.
9.
Monteagudo, J. M.; Durán, A.; San Martin, I.; Carnicer, A.,
Appl. Catal., B 2011, 106, 242–249.
10. Braslavsky, S. E.; Braun, A. M.; Cassano, A. E.; Emeline, A.
V.; Litter, M. I.; Palmisano, L.; Parmon, V. N.; Serpone, N., Pure
Appl. Chem. 2011, 83, 931–1014.
11. (a) Houas, A.; Lachheb, H.; Ksibi, M.; Elaloui, E.; Guillard, C.;
Herrmann, J.ꢀM., Appl. Catal., B 2001, 31, 145–157; (b) Yang, J.;
Dai, J.; Chen, C.; Zhao, J., J. Photochem. Photobiol., A 2009, 208, 66ꢀ
77; (c) Murakami, Y.; Endo, K.; Ohta, I.; Nosaka, A. Y.; Nosaka, Y.,
J. Phys. Chem. C 2007, 111, 11339ꢀ11346; (d) Sun, L.; Bolton, J. R.,
J. Phys. Chem. 1996, 100, 4127–4135.
12. Tachikawa, T.; Majima, T., J. Fluoresc. 2007, 17, 727–738.
13. (a) Daimon, T.; Hirakawa, T.; Kitazawa, M.; Suetake, J.;
Nosaka, Y., Appl. Catal., A 2008, 340, 169–175; (b) Daimon, T.;
Nosaka, Y., J. Phys. Chem. C 2007, 111, 4420–4424; (c) Nosaka, Y.;
Daimon, T.; Nosaka, A. Y.; Murakami, Y., Phys. Chem. Chem. Phys.
2004, 6, 2917–2918.
14. Buchalska, M.; Łabuz, P.; Bujak, Ł.; Szewczyk, G.; Sarna, T.;
Maćkowski, S.; Macyk, W., Dalton Trans. 2013, 42, 9468ꢀ9475.
15. Ohtani, B.; PrietoꢀMahaney, O. O.; D. Li, R. A., J. Photochem.
Photobiol., A 2010, 216, 179–182.
Excitation of rꢀTiO2/aꢀTiO2 composites leads to a desirable
charge separation, involving the electron transfer from rꢀTiO2
to aꢀTiO2 (confirmed by the studies of Ohno, compare Introꢁ
duction). Due to the presence of rꢀTiO2 the amount of oxygen
adsorbed at the composite is improved when compared to aꢀ
TiO2. On the other hand, the oxidation properties of rꢀTiO2/aꢀ
TiO2 are better than of bare rꢀTiO2. The overall photocatalytic
activity of the composite can be optimized by tuning the anaꢀ
tase/rutile ratio.
The analysis presented in this paper sheds light on the difꢀ
ferences in photocatalytic activity of two most common forms
of titanium dioxide and their composites. More detailed studꢀ
ies should take into account various features of specific crystal
ACS Paragon Plus Environment

Page 9 of 10
ACS Catalysis
16. (a) Ohno, T.; Tokieda, K.; Higashida, S.; Matsumura, M., Appl.
26. (a) Thuan, D. B.; Kimura, A.; Ikeda, S.; Matsumura, M., J. Am.
Chem. Soc. 2010, 132, 8453–8458; (b) Pang, X. B.; Chen, C. C.; Ji,
H. W.; Che, Y. K.; Ma, W. H.; Zhao, J. C., Molecules 2014, 19,
16291ꢀ16311.
27. Augugliaro, V.; Loddo, V.; LópezꢀMuñnoz, M. J.; Márquezꢀ
Álvarez, C.; Palmisano, G.; Palmisano, L.; Yurdakal, S., Photochem.
Photobiol. Sci. 2009, 8, 663–669.
28. Li, C.ꢀJ.; Xu, G.ꢀR.; Zhang, B.; Gong, J. R., Appl. Catal., B
2012, 115ꢁ116, 201–208.
29. Matsushita, M.; Tran, T. H.; Nosaka, A. Y.; Nosaka, Y., Catal.
Today 2007, 120, 240–244.
30. (a) Soana, F.; Sturini, M.; Cermenati, L.; Albini, A., J. Chem.
Soc., Perkin Trans. 2 2000, 2, 699ꢀ704; (b) Theurich, J.; Bahnemann,
D. W.; Vogel, R.; Ehamed, F. E.; Alhakimi, G.; Rajab, I., Res. Chem.
lntermed 1997, 23, 247–274.
31. Buchalska, M.; Pacia, M.; Kobielusz, M.; Surówka, M.;
Świętek, E.; Wlaźlak, E.; Szaciłowski, K.; Macyk, W., J. Phys. Chem.
C 2014, 118, 24915–24924.
Catal., A 2003, 244, 383–391; (b) Hurum, D. C.; Agrios, A. G.; Crist,
S. E.; Gray, K. A.; Rajh, T.; Thurnauer, M. C., J. Electron Spectrosc.
Relat. Phenom. 2006, 150, 155ꢀ163; (c) Hurum, D. C.; Agrios, A. G.;
Gray, K. A.; Rajh, T.; Thurnauer, M. C., J. Phys. Chem. B 2003, 107,
4545–4549; (d) Kawahara, T.; Konishi, Y.; Tada, H.; Tohge, N.;
Nishii, J.; Ito, S., Angew. Chem., Int. Ed. 2002, 41, 2811–2813; (e)
Miyagi, T.; Kamei, M.; Mitsuhashi, T.; Ishigaki, T.; Yamazaki, A.,
Chem. Phys. Lett. 2004, 390, 399–402; (f) Komaguchi, K.; Nakano,
H.; Araki, A.; Harima, Y., Chem. Phys. Lett. 2006, 428, 338–342; (g)
Kisch, H., Semiconductor Photocatalysis: Principles and
Applications. WileyꢀVCH: Weinheim, 2015; p 264; (h) Scanlon, D.
O.; Dunnill, C. W.; Buckeridge, J.; Shevlin, S. A.; Logsdail, A. J.;
Woodley, S. M.; Catlow, C. R. A.; Powell, M. J.; Palgrave, R. G.;
Parkin, I. P.; Watson, G. W.; Keal, T. W.; Sherwood, P.; Walsh, A.;
Sokol, A. A., Nat. Mater. 2013, 12, 798–801.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
17. Rao, M. V.; Rajeshwar, K.; Paiverneker, V. R.; Dubow, J., J.
Phys. Chem. 1980, 84, 1987–1991.
18. (a) Masahashi, N.; Mizukoshi, Y.; Semboshi, S.; Ohtsu, N.,
Appl. Catal., B 2009, 90, 255–261; (b) Arbuj, S. S.; Hawaldar, R. R.;
Mulik, U. P.; Wani, B. N.; Amalnerkar, D. P.; Waghmode, S. B.,
Mater. Sci. Eng., B 2010, 168, 90–94; (c) Jaimy, K. B.; Baiju, K. V.;
Ghosh, S.; Warrier, K. G. K., J. Solid State Chem. 2012, 186, 149ꢀ
157.
19. Bernardini, C.; Cappelletti, G.; Dozzi, M. V.; Selli, E., J.
Photochem. Photobiol., A 2010, 211, 185–192.
20. (a) Sonawane, R. S.; Ramakrishna, S., Mater. Sci. Eng., B 2012,
177, 652ꢀ660; (b) Gao, Y. N.; Wang, H.; Wu, J.; Zhao, R. H.; Lu, Y.
F.; Xin, B. F., Appl. Surf. Sci. 2014, 294, 36–41.
21. Kawahara, T.; Ozawa, T.; Iwasaki, M.; Tada, H.; Ito, S., J.
Colloid Interface Sci. 2003, 267, 377–382.
22. Addamo, M.; Bellardita, M.; Di Paola, A.; Palmisano, L.,
Chem. Commun. 2006, 47, 4943–4945.
23. Tayade, R. J.; Surolia, P. K.; Kulkarni, R. G.; Jasra, R. V., Sci.
Technol. Adv. Mater. 2007, 8, 455–463.
24. Hirakawa, T.; Yawata, K.; Nosaka, Y., Appl. Catal., A 2007,
325, 105ꢀ111.
32. Świętek, E.; Pilarczyk, K.; Derdzińska, J.; Szaciłowski, K.;
Macyk, W., Phys. Chem. Chem. Phys. 2013, 15, 14256–14261.
33. Hirakawa, T.; Nosaka, Y., Langmuir 2002, 18, 3247–3254.
34. Gong, X.ꢀQ.; Selloni, A.; Batzill, M.; Diebold, U., Nature
materials 2006, 5, 665–670.
35. Yu, J. C.; Lin, J.; Lo, D.; Lam, S. K., Langmuir 2000, 16,
7304–7308.
36. Tsujiko, A.; Itoh, H.; Kisumi, T.; Shiga, A.; Murakoshi, K.;
Nakato, Y., J. Phys. Chem. B 2002, 106, 5878–5885.
37. Li, R.; Weng, Y.; Zhou, X.; Wang, X.; Mi, Y.; Chong, R.; Han,
H.; Li, C., Energy Environ. Sci. 2015, 8, 2377–2382.
38. Jankulovska, M.; Berger, T.; LanaꢀVillarreal, T.; Gómez, R.,
Electrochim. Acta 2012, 62, 172–180.
39. Redmond, G.; Fitzmaurice, D., J. Phys. Chem. 1993, 97, 1426–
1430.
40. Morris, A. J.; Meyer, G. J., J. Phys. Chem. C 2008, 112,
18224–18231.
41. Nakabayashi, Y.; Nosaka, Y., J. Phys. Chem. C 2013, 117,
23832–23839.
25. (a) Cermenati, L.; Dondi, D.; Fagnoni, M.; Albini, A.,
Tetrahedron 2003, 59, 6409–6414; (b) Ohno, T.; Mitsui, T.;
Matsumura, M., J. Photochem. Photobiol., A 2003, 160, 3ꢀ9.
ACS Paragon Plus Environment

ACS Catalysis
Page 10 of 10
1
2
3
Table of Contents artwork
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
10
ACS Paragon Plus Environment